Foil style switch elements, such as membrane switches, foil pressure switches or similar elements, in general comprise at least two essentially elastic foil layers positioned at a certain distance from one another. This is effected, for example, by means of a spacer which is arranged around the active region of the switch element and to which the two foil layers are glued with their respective edges. In the active region of the switch element, various contact arrangements are attached to the foil layers, between which, when the two foil layers are pressed together, an electric contact is created, so that the switch element is triggered. When the pressure is released from the foil layers, these in turn are, due to their elasticity, restored to their spaced position and the electric contact between the various contact arrangements is interrupted.
Such a switch element has a very low assembly height and is in particular characterized by the multifarious possibilities of designing the command button. This makes such switch elements particular suitable for the use in fields where small structural dimensions and a flexible design of the command buttons is required.
When using switch elements in fields where varying light conditions have to be expected, switch elements are preferably designed such that their command button to be seen from the outside is illuminated from behind. With conventional switches, this background illumination often is achieved by the switch key of the switch being at least partly manufactured from transparent plastics and illuminated from behind by a filament lamp. Alternatively, the assembly space behind the switch key can be lightened via an optical fiber. Such an embodiment of the switch illumination, however, cannot be employed in a foil style switch element due to the desired low assembly height.
The document U.S. Pat. No. 4,812,831 describes an illuminated foil keyboard where a foil luminescent screen is applied on a foil keyboard. The foil keyboard comprises an upper carrier foil and a lower carrier foil being arranged at a certain distance from one another by means of a spacer. On the lower carrier foil, two electrode structures are applied in a spaced manner, which are contacted on actuation of the switch element by a layer of a conductive material applied on the bottom side of the upper carrier foil. On the top side of the upper carrier foil, first a screening grid of an electrically conductive ink is applied. On this screening grid, an adhesive layer is applied by which the luminescent screen unit is glued to the keyboard. The luminescent screen unit comprises a luminescent matter layer of a phosphorous material applied on a polyester layer and a plurality of electrodes arranged at the bottom side of this polyester layer. Above the luminescent screen unit, a further foil inscribed with graphic symbols is arranged by means of an adhesive layer.
The document EP-A-0 996 131 describes an illuminated switch element having two carrier foils laminated together, between which contact arrangements are attached such that an electric contact between the contact arrangements is created when the carrier foils are pressed together. On the upper carrier foil, a luminescent screen unit is laminated by means of a spacer of a double-sided bonding sheet. The luminescent screen unit itself comprises a lower insulating foil, a first electrode layer, a dielectric layer, a luminescent matter layer, a second electrode layer and an upper transparent insulating layer which are applied to one another in this order.
The document U.S. Pat. No. 6,069,444 describes an input device having a printed circuit board on which a plurality of contact arrangements are applied.
Above the printed circuit board, a switching layer having several triggering elements is arranged, such that at least two of the contact arrangements are short-circuited by a triggering element when a vertical operating force strikes it. Above the switching layer, a luminescent screen unit is arranged. This luminescent screen unit comprises a lower carrier foil on which connection lines for a plurality of luminescent units are applied. The luminescent units comprise each from top to bottom a transparent carrier layer, a first transparent electrode layer, a luminescent matter layer, a dielectric layer and a lower electrode layer.
The document DE A-0 763 838 describes a membrane switch having a dome spring where a luminescent screen unit is applied within the dome spring on the top side of an insulating foil. The luminescent screen unit comprises a phosphorous layer and a dielectric layer.